Often seen basking in somewhat untidy piles on the beach (see below!), the Galapagos sea lion (Zalophus wollebaeki) is a highlight for many visitors to the Galapagos Islands. Sociable and playful by nature, they provide hours of entertainment for adoring onlookers as they move in their ungainly waddle on land. But get them in the water and sea lions transform into 400 kg living torpedoes capable of moving with the elegance and agility of a ballet dancer. Feeding primarily on sardines, Galapagos sea lions are experts at manoeuvring and hunting underwater. They have been recorded to dive to depths of around 350 metres and can stay submerged for almost ten minutes. So how do they do it? It all comes down to something called the mammalian dive reflex.
The mammalian dive reflex is an innate reflex exhibited most strongly in marine mammals, but is also present in other mammals including humans. It occurs when the face of the animal comes into contact with water that is 20⁰C (68⁰F) or colder, and it triggers a series of physiological changes within the body which together allow a mammal to stay submerged for a long time.
The primary limitation when it comes to holding your breath is the body’s requirement for oxygen. Oxygen is used in respiration, an essential process which keeps us and all animals alive, and which helps to provide us with energy. Reducing oxygen usage within the body results in an animal needing to take less frequent breaths and hence they are able to stay underwater for longer.
The first change associated with the dive reflex is bradycardia: a slowing of the heart rate. A heart that is beating slowly requires less oxygen than one that is beating fast. In humans, the dive reflex can reduce the heart rate by 10 to 25% in just 20 seconds. In sea lions, diving bradycardia is much stronger and can reduce the heart rate from around 125 beats per minute to just ten.
The next response to kick-in is termed peripheral vasoconstriction, where capillaries in the extremities close. By reducing blood flow to the extremities, more blood, and therefore more oxygen, is available for use by vital organs such as the heart and brain. In humans, capillaries in the fingers and toes are the first to close, followed by the hands and feet and eventually the arms and legs. The consequence of this is that the muscles within these extremities are not resupplied with oxygen until the capillaries have opened again, resulting in lactic acid building up which causes cramp. Some oxygen is stored within the muscles, bound to a protein called myoglobin, but in humans this only accounts for 12% of the body’s oxygen. In sea lions however, 25% of the animal’s oxygen is stored this way, resulting in their muscles being able to function for a lot longer than a human’s and allowing for a longer dive.
A third response is splenic contraction. Diving mammals such as seals and sea lions have large spleens which are capable of storing blood (up to 20 litres in the Weddell Seal), therefore acting as an oxygen reservoir. When the spleen becomes contracted during a dive, the number of oxygen-carrying red blood cells in the circulatory system of a sea lion can increase by up to 10%, again resulting in the animal being able to stay underwater for longer.
These adaptations, along with certain others, make diving mammals excellently suited to an aquatic lifestyle, and the Galapagos sea lion is no exception!
by Pete Haskell